Non-redundant Roles of Phosphoinositide 3-Kinase Isoforms a and b in Glycoprotein VI-induced Platelet Signaling and Thrombus Formation

• Published in: The Journal of biological chemistry 2009-12, Vol.284 (49)
• Main Authors: Gilio, Karen, Munnix, Imke CA, Mangin, Pierre, Cosemans, Judith MEM, Feijge, Marion AH, Van der Meijden, Paola EJ, Olieslagers, Serve, Chrzanowska-Wodnicka, Magdalena B, Lillian, Rivka, Schoenwaelder, Simone, Koyasu, Shigeo, Sage, Stewart O, Jackson, Shaun P, Heemskerk, Johan WM
• Format: Article
• Language: English
• ISSN: 0021-9258
• DOI: 10.1074/jbc.M109.048439

Platelets are activated by adhesion to vascular collagen via the immunoglobulin receptor, glycoprotein VI (GPVI). This causes potent signaling toward activation of phospholipase Cg2, which bears similarity to the signaling pathway evoked by T- and B-cell receptors. Phosphoinositide 3-kinase (PI3K) plays an important role in collagen-induced platelet activation, because this activity modulates the autocrine effects of secreted ADP. Here, we identified the PI3K isoforms directly downstream of GPVI in human and mouse platelets and determined their role in GPVI-dependent thrombus formation. The targeting of platelet PI3Ka or -b strongly and selectively suppressed GPVI-induced Ca super(2+) mobilization and inositol 1,4,5-triphosphate production, thus demonstrating enhancement of phospholipase Cg2 by PI3Ka/b. That PI3Ka and -b have a non-redundant function in GPVI-induced platelet activation and thrombus formation was concluded from measurements of: (i) serine phosphorylation of Akt, (ii) dense granule secretion, (iii) intracellular Ca super(2+) increases and surface expression of phosphatidylserine under flow, and (iv) thrombus formation, under conditions where PI3Ka/b was blocked or p85a was deficient. In contrast, GPVI-induced platelet activation was insensitive to inhibition or deficiency of PI3Kd or -g. Furthermore, PI3Ka/b, but not PI3Kg, contributed to GPVI-induced Rap1b activation and, surprisingly, also to Rap1b-independent platelet activation via GPVI. Together, these findings demonstrate that both PI3Ka and -b isoforms are required for full GPVI-dependent platelet Ca super(2+) signaling and thrombus formation, partly independently of Rap1b. This provides a new mechanistic explanation for the anti-thrombotic effect of PI3K inhibition and makes PI3Ka an interesting new target for anti-platelet therapy.